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United States Patent |
5,754,414
|
Hanington
|
May 19, 1998
|
Self-compensating switching power converter
Abstract
A self-compensating high voltage switched power converter monitors the
variations in real time of the resonant frequency of the converter, and
controls a switching transistor of the converter to establish an operating
frequency which corresponds to the resonant frequency. The collector
voltage of the switching transistor is monitored, and the transistor is
switched only when the collector voltage is decreasing toward a minimum
value and is below a predetermined reference level. This enables the power
converter to operate at a high frequency, which affords small size, light
weight, and high efficiency.
Inventors:
|
Hanington; Gary J. (4411 Willows Rd., Alpine, CA 90901)
|
Appl. No.:
|
606147 |
Filed:
|
February 23, 1996 |
Current U.S. Class: |
363/21.12; 323/282; 363/17; 363/21.01 |
Intern'l Class: |
H02M 003/335; G05F 001/563 |
Field of Search: |
363/21,17,98
323/282,285,280
|
References Cited
U.S. Patent Documents
4616300 | Oct., 1986 | Santelmann, Jr. | 363/21.
|
4725936 | Feb., 1988 | Nakajima et al. | 363/21.
|
4823070 | Apr., 1989 | Nelson | 323/285.
|
5291387 | Mar., 1994 | Oshima | 363/56.
|
5305192 | Apr., 1994 | Bonte et al. | 363/21.
|
Primary Examiner: Nappi; Robert
Assistant Examiner: Patel; Rajnikant B.
Attorney, Agent or Firm: Gray Cary Ware & Freidenrich
Claims
I claim:
1. An apparatus for controlling a switching circuit driving a load, the
circuit including a switching device that switches between on and off
operating states, comprising means for monitoring a device voltage across
the switching device, said device voltage exhibiting transient variations
having peak values and minimum values about a supply voltage upon said
switching and having a frequency which depends on a resonant
characteristic of the switching circuit and the load; and means for
controlling the switching device to switch the switching device on when
said device voltage decreases from a peak value towards a minimum value.
2. The apparatus of claim 1 further comprising means for establishing
another reference voltage, and wherein the control means switches the
switching device between states only when said device voltage decreases
below both of said reference voltages.
3. The apparatus of claim 2, wherein said reference voltages are
established such that one reference voltage is above a nominal supply
voltage and the other reference voltage is below said nominal supply
voltage.
4. The apparatus of claim 3, further comprising means for producing said
reference voltages so that their relationship to one another remains
substantially constant with variations in the supply voltage.
5. The apparatus of claim 1, wherein said apparatus comprises a power
converter for supplying power, the power converter having transformer
means for transforming a first voltage to an output voltage, and wherein
said switching device comprises a switching transistor connected to said
transformer means for alternately enabling current flow and for
interrupting current flow through said transformer means in accordance
with its operating state.
6. The apparatus of claim 5, wherein said device voltage comprises the
collector voltage of the switching transistor, and wherein said transient
characteristics of said collector voltage have a time-varying ringing
characteristic and a frequency related to a resonant frequency of the
converter.
7. The apparatus of claim 1, wherein said switching device switches when
said device voltage is below said predetermined reference voltage and is
decreasing in value.
8. The apparatus of claim 1, wherein the transient variations of said
device voltage has damped oscillations about said predetermined voltage
and said switching device switches when said device voltage first changes
from an initial peak value to said first-mentioned predetermined reference
voltage in order to determine a conduction time for said switching device.
9. The apparatus of claim 8, wherein said operating states of said
switching device comprise a conducting state and a non-conducting state,
said switching device switching to said conducting state upon said device
voltage being below said first-mentioned reference voltage and decreasing
in value and switching to said non-conducting state when said device
voltage is above said first-mentioned reference voltage.
10. The apparatus of claim 1, wherein said frequency varies in accordance
with changes in the load and said means for controlling said switching
device adjusts to the variations in said frequency.
11. A high voltage power converter comprising means for transforming a
primary time-varying voltage to a higher secondary time-varying voltage
driving a load; switching means connected to the transforming means for
switching current flow to the transforming means on and off to produce
said primary time-varying voltage; means for monitoring a device voltage
across said switching means upon switching, said device voltage exhibiting
transient variations having peak values and minimum values about a supply
voltage and having a frequency which depends upon a resonant
characteristic of the switching circuit and the load; and means,
responsive to the monitoring means, for controlling the switching means to
switch on when said device voltage decreases from a peak value towards a
minimum value.
12. The converter of claim 11, further comprising means for establishing
first and second reference voltages about a nominal operating voltage, and
wherein said controlling means controls the switching means to switch
states when said switching voltage decreases below both said first and
second reference voltages.
13. The converter of claim 12, wherein said device voltage has a periodic
time-varying characteristic, and wherein said controlling means controls
the switching means to switch states when said device voltage has a phase
angle on the order of 10.degree. below said nominal operating voltage.
14. The converter of claim 12 further comprising voltage divider means for
establishing said first and second reference voltages with respect to said
nominal operating voltage so that the relationship between said reference
voltages remains substantially constant with variations in the nominal
operating voltage.
15. The converter of claim 11, wherein said device voltage has a transient
ringing characteristic, and wherein said control means controls the
switching means to change states when the device voltage first decreases
from an initial peak value to said predetermined value in order to achieve
a high power transfer efficiency.
16. The apparatus of claim 11, wherein said switching device switches when
said device voltage is below said predetermined reference voltage and is
decreasing in value.
17. A method of controlling a switched power converter attached in which a
switching device is switched between first and second states to apply
power to an output circuit, comprising monitoring a voltage across the
switching device, the device voltage exhibiting transient variations
having peak values and minimum values about a supply voltage upon said
switching and having a frequency which depends on a resonant
characteristic of the switching circuit and the output circuit, and
controlling the switching device to switch on when said device voltage
decreases from a peak value towards a minimum value.
18. The method of claim 17, wherein said switching device switches when
said device voltage is below said predetermined reference voltage and is
decreasing in value.
19. The method of claim 17 further comprising establishing another
reference voltage, and controlling the switching device to switch when
said associated voltage decreases below both of said reference voltages.
20. The method of claim 19, wherein said reference voltages are established
above and below a nominal supply voltage, and are referenced to said
supply voltage such that the relationship between the reference voltages
remain substantially constant during variations in said supply voltage.
21. The method of claim 20, wherein said switching device is controlled to
switch states when said device voltage has a phase angle of the order of
10.degree. relative to the supply voltage.
22. The method of claim 17, wherein controlling said switching device
comprises changing the states of the switching device when the device
voltage first decreases from an initial peak value to said predetermined
value in order to afford a high switching frequency.
23. An apparatus for controlling a switching circuit driving a load, the
circuit including a switching device that switches between first and
second operating states, comprising means for monitoring a device voltage
across the switching device, said device voltage exhibiting transient
variations upon said switching having a resonant frequency which depends
on a resonant characteristic of the switching circuit and the load and
varies in response to changes in the load; and means for controlling the
switching device to switch between operating states at the resonant
frequency .
Description
BACKGROUND OF THE INVENTION
This invention relates generally to high voltage switching circuits, and
more particularly to switching DC--DC high voltage power converters which
are particularly useful, for example, for supplying power to cathode ray
tubes, flat screen display devices, or other high voltage devices such as
travelling wave tubes or X-Ray tubes.
Early high voltage power supplies were linear devices based upon the Royer
design. These devices utilized a DC voltage to drive an oscillator which
was connected to a high voltage transformer, the secondary of which
supplied high voltage rectifiers and filters to produce the desired output
DC voltage. These devices had relatively low power efficiency, in the
order of 50-60%, and were rather large and bulky. This made them
unsuitable for portable devices or for applications, such as in military
aircraft, where weight and size are critical.
Advances in transistor reliability, integrated circuit drivers and
magnetics, led to the abandonment of the inefficient linear designs in
favor of driven switched high voltage power converters. Driven switched
systems use integrated circuits to turn a switching transistor on and off
to produce a time-varying voltage across the primary of a high voltage
transformer. The secondary of the transformer, as in the linear design, is
coupled to a high voltage rectifier and filter, and feedback is supplied
to the switch driver for regulation purposes.
Switching high voltage power supplies operate at a relatively low frequency
of the order of 30 kHz. The low frequency has been dictated by the
necessity of allowing high voltage transients to die out before switching
the transistor, in order to prevent damage or destruction of the
transistor and other circuit components. The relatively low operating
frequency necessitates rather large and bulky components. They are not
well suited for battery operation, and their large size is still
undesirable for either portable devices or where weight and size are
important considerations.
It is not possible, with conventional switching power converters, to reduce
the size of the inductors and capacitors to produce a lighter and smaller
device, since this increases the operating frequency of the device, and
increases the possibility of destruction of the switching transistor by
causing it to operate during high voltage transients. Unfortunately, the
resonant frequency of a switching power supply, which determines the
transient response, is a function of many different variables, including
load, temperature, and supply voltage.
While driving a conventional switched converter at a fixed frequency may
work fine at one operating condition, the resonant-point will shift with
load variations. A heavier load is required as when, for example, a
display produces an all white raster. The result is that the resonant
frequency decreases, and the operating frequency would thus have to be
lower. As a result, conventional high voltage switching power converters
have very low efficiencies over their normal operating range.
It is desirable to provide high voltage power supplies which can operate at
high frequencies, in order to reduce their size and weight, by allowing a
reduction in values and sizes of inductors and capacitors of the system,
and which are capable of operating with high efficiency and at relatively
low DC voltages such as used in portable integrated circuit devices. It is
to these ends that the present invention is directed.
SUMMARY OF THE INVENTION.
The invention provides self-compensating resonant switching high voltage
power converters which solve the foregoing and other problems of known
high voltage power supplies. The power converters of the present invention
achieve high efficiency over widely varying operating conditions, such as
load, temperature and supply voltage, while having low weight and size.
They are able to operate efficiently with low conventional battery
voltages such as used in portable devices. The invention achieves these
advantages, in part, by operating at a high frequency which is
automatically adjusted in real time in accordance with variations in
operating conditions in order to ensure that the switching devices always
operates at the optimum point.
The invention automatically compensates for variations in input supply
voltage, load and temperature, by sensing the actual voltage of the
switching devices, and by controlling the switching to turn on or off at
the appropriate time. This effectively tracks the resonant frequency of
the converter in real time, and automatically adjusts its operating
frequency to match. As a result, the invention can operate at high
frequencies of the order of 300 Khz or higher, a factor of ten times the
operating frequency of conventional fixed frequencies in converters. This
results in a substantial size and weight reduction, of the order of five
times, for example, making the invention ideally suited for providing
power to displays in portable laptop computers, on military or other
aircraft, or in any high voltage application where it is desirable to
reduce the size and weight of the power supply.
The invention advantageously always seeks and operates at the resonant
frequency of the circuit, and follows changes in resonant frequency with
changes in operating loads and conditions. Moreover, the circuit employs a
pulse width modulator type of driver, which is very efficient and very
fast correcting. Moreover, the power converter of the invention will
always start into any load, such as large capacitive loads which would
cripple a linear design, and at any temperature, even where the beta of
the switching transistor is extremely low.
In one aspect, the invention affords an apparatus and method for
controlling a switching circuit which includes a switching device that
switches between first and second operating states in which a voltage
associated with the switching device that exhibits transient variations
during switching is monitored, and the switching device is controlled to
switch between operating states when the associated voltage obtains a
predetermined relationship to a reference voltage.
More specifically, the switching device is controlled to switch between
operating states when the associated voltage is decreasing toward a
minimum value and is below a nominal operating voltage, and preferably
when the associated voltage first decreases from a maximum to the
predetermined relationship to the reference voltage.
In another aspect, the invention affords a high voltage power converter
which includes means for transforming a primary time-varying voltage to a
higher secondary time-varying voltage, in which transistor means connected
to the transforming means is controlled to switch the current flow through
the transformer means on and off to produce the time-varying voltage, and
the switching voltage of the transistor means is monitored to control the
transistor means to switch between states when the switching voltage
attains a predetermined value.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic and block diagram illustrating a conventional
fixed-frequency high voltage switching power converter circuit.
FIGS. 2A and 2B (together FIG. 2) illustrate voltage waveforms at certain
locations in the circuit of FIG. 1;
FIG. 3 illustrates a self-compensating resonant high voltage switching
power converter in accordance with the invention;
FIGS. 4A-D (collectively FIG. 4) illustrate voltage waveforms as a function
of time at various points in the circuit of FIG. 3; and
FIGS. 5, 5a, and 5b are a more detailed schematic diagram of a
self-compensating resonant high voltage switched power converter in
accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS.
The invention is particularly applicable to high voltage power converters
for supplying power to, for example, displays, in portable devices, and
for use on aircraft, and will be described in that context. It will be
appreciated, however, from the description which follows that the
invention has broader utility to other high voltage switching circuits and
devices.
FIGS. 1 and 2 illustrate a conventional fixed frequency switching power
converter of a known type, and will be described briefly since this will
facilitate an understanding of the invention. As shown in FIG. 1, the
principal components of a conventional power converter 10 comprise a high
voltage transformer 12 having one side of its primary winding 14 connected
to the supply voltage V.sub.s and the other side of the primary winding
connected to the collector of a switching transistor 18. The emitter 20 of
the switching transistor may be connected to ground, and the base 22 of
the transistor may be driven by an integrated circuit (IC) driver 24. IC
24 may also be connected to the supply voltage V.sub.s, and is used to
turn switching transistor 18 on and off in order to produce a time-varying
voltage across the primary of the transformer. The secondary 30 of the
transformer supplies a high voltage rectifier 32, the output of which is
connected to a filter, such as a capacitor 34 and supplies an output
voltage V.sub.o to a load 40.
High voltage transformer 12 may have a step-up turns ratio, n, of the order
of 100-1,000, for example, thereby converting a rather low peak-to-peak
voltage across the transformer primary to a high output peak-to-peak
voltage. The rectifier 32 converts the voltage across the transformer
secondary winding to a uni-directional voltage, and the filter 34 smooths
variations to produce a high voltage DC output V.sub.0. A portion of the
high voltage may be fed back via a feedback path 42 to IC driver 24 for
voltage regulation purposes. IC driver 24 may be a conventional integrated
circuit which produces an output pulse at a frequency controlled by a
timing circuit comprising a resistor, R, and a capacitor, C. The timing
circuit sets the operating frequency of the switching converter 10.
The operation of the switch converter 10 is illustrated by the waveforms of
FIGS. 2. FIG. 2A illustrates the collector voltage Vc of transistor 18 as
a function of time, and FIG. 2B illustrates the base voltage Vb of the
transistor.
Referring to FIG. 2A, between the times t.sub.0 -t.sub.1, it is assumed
that transistor 18 is off. Accordingly, the voltage at its collector
V.sub.c is equal to the supply voltage V.sub.s. At t.sub.1, a voltage
pulse V.sub.b is applied to the base of the transistor by the IC driver
24. This forward biases the base-emitter junction of the transistor,
turning it on at time t.sub.1. When the transistor turns on, its collector
voltage V.sub.c drops to approximately zero, as shown in FIG. 2A. Between
times t.sub.1 -t.sub.2, the IC driver 24 applies the voltage pulse to the
transistor base 22, maintaining the transistor in a conductive state. This
occurs by the IC driver outputting a voltage pulse to the base having a
pulse width of t.sub.2 -t.sub.1, as shown in FIG. 2B. At t.sub.2,the
output pulse from the IC driver goes to zero, causing the base voltage to
likewise go to zero, and turns the transistor off. This interrupts the
current flow through the primary of the transformer, producing an induced
voltage V.sub.i =L(di/dt), where L is the inductance and di/dt is the rate
of change of current through the primary as a function of time. This is
illustrated in FIG. 2A where, at time t.sub.2 the collector voltage
V.sub.c jumps to a voltage V.sub.i, which may be substantially higher
(about twice, for example) than the supply voltage V.sub.s. The induced
voltage then begins to decay as a periodic time-varying voltage, as shown
at 44, to a level equal to the supply voltage, producing damped ringing
and transient effects as illustrated.
The rate of decay of the voltage 44 at the collector of the transistor is a
function of a number of different parameters, including the load, the
temperature, and the supply voltage, and the decay is greatly affected by
these parameters. As illustrated in FIG. 2A, under normal operating
conditions, the rate of decay may vary from a fast decay, as shown by the
dotted line 46, to a substantially longer delay, as illustrated by the
dotted line 48. In order to accommodate such widely varying decay rates as
may be encountered in normal operations, it has been necessary in
conventional switching converters to hold the transistor in an off
condition for a sufficiently long period of time to ensure that the
transient effects associated with the longest anticipated decay,
corresponding to the lowest resonant frequency, have died out. This is to
avoid turning the transistor on at a time when its collector voltage is
either at a high level or increasing, which can damage or destroy the
transistor. Thus, the operating frequency of the IC driver had to be set
to produce an operating period, T, corresponding to the longest
anticipated time to enable the transient effects to die out. This is
illustrated in FIG. 2A as the time interval T between t.sub.1 and t.sub.3.
At t.sub.3, the IC driver outputs a second pulse, as shown in FIG. 2B, to
turn the transistor on again until t.sub.4, and the cycle repeats. With
such conventional switching power converters, the operating frequency,
must typically be set rather low, e.g., 30-40 kHz, in order to provide a
sufficiently long time between operating cycles to accommodate the
anticipated variations in transient response of the switching converter
due to variations in operating parameters. The necessity of operating at a
lower frequency, f, than desirable results in inefficiency as well as
large size and weight because of the size and weight of the magnetics and
other components required.
The invention avoids such problems of conventional fixed-frequency power
converters by providing a new self-compensating resonant frequency
switching power converter that automatically adjusts its frequency of
operation dynamically and in real time in order to operate at the resonant
frequency of the converter circuit as it varies during its actual
operating conditions. This enables the invention to operate at a
substantially higher frequency, e.g., 300 kHz, which is a factor of 10
higher than a conventional switching power converter. This is possible
since the rather long voltage decays illustrated in FIG. 2A which dictate
the long cycle time and low frequency of operation of a conventional
switching converter occur rather infrequently, but nevertheless must be
accommodated in a conventional device. Since the invention actually tracks
the resonant frequency and adjusts its operation accordingly, it is able
to operate at sustained high frequencies for a substantial part of the
time, thereby providing substantially higher operating efficiency.
As will be described shortly, the invention also employs a pulse width
modulator type of driver, which is a very efficient and very fast
correcting driver. Moreover, a power converter in accordance with the
invention will always start into any load, even large capacative loads
which would cripple a linear supply or conventional switching supply and
over wide temperature and supply voltage ranges. Significantly, the
invention can operate with very high efficiency at low battery voltages of
the order of 3.3 volts, for example, which are used in some of the newer
integrated circuit devices. This is something that conventional devices
cannot do.
The problem addressed by the invention may be analogized to the operation
of a mass, i.e., block, suspended by a simple spring. When the mass is
pulled down and released, it undergoes an oscillatory motion, up and down,
with a damped frequency determined by the spring constant and the mass of
the block itself. The best place in the cycle to add energy to such a
system is to pull the mass down while it is naturally heading in that
direction during part of its cycle. It would be inefficient to tug the
mass downwardly to add energy as it was heading upwardly. This is
analogous to what happens in a conventional high voltage switching power
supply where the driver is out of synchronization with the needs of the
resonant circuit. As will now be described, the invention monitors the
actual operating condition, and adds energy to the system at the
appropriate time to achieve high efficiency and high operating frequency.
FIG. 3 illustrates a high voltage switching power converter 60 embodying
the invention. As with a conventional fixed frequency switching converter,
a switching device, such as a transistor 18 or other voltage controlled
switch, e.g., an SCR, drives a high voltage step-up transformer 12. The
secondary winding 30 of the transformer supplies a high voltage rectifier
32 and filter capacitor 34 to provide the output voltage V.sub.0. A
voltage divider 62 samples the output voltage and feeds back the sample
voltage to a voltage regulator 64, which may comprise an operational
amplifier (op amp). The sample voltage from the voltage divider 62 may be
provided to the inverting (-) terminal of the op amp, and a reference
voltage V.sub.ref may be supplied to the non-inverting (+) terminal of the
op amp. The output of the op amp supplies a voltage V.sub.reg to a switch
driver 70 for switching transistor 18. As will be described shortly, the
output voltage V.sub.reg provides a separate control signal to the switch
driver 70 to regulate the output V.sub.0 from the high voltage regulator
to a predetermined value. Driver 70 may compromise a comparator which
compares a control voltage in the form of a ramp (V.sub.ramp) to the
output voltage from the voltage regulator to turn the switching transistor
18 on and off.
In accordance with the invention, a sensing and controlling circuit 80 is
provided for monitoring the collector voltage V.sub.c of switching
transistor 18, and for controlling the switch driver 70 so that power is
added by switching transistor 18 only when the collector voltage is in a
predetermined voltage range. The sensing and control circuit 80 preferably
prevents switching transistor 18 from turning on to add power whenever the
collector voltage is increasing or above a predetermined value. This
prevents the transistor from attempting to add power while the ringing
voltage induced across the transistor primary, as described in connection
with FIG. 2, is high and could cause damage or destruction to the
switching transistor or other components of the power converter. Ideally,
it is desirable to turn the switching transistor on when the collector
voltage is decreasing below the level of the supply voltage during
ringing. This corresponds to the region indicated at 82 in FIG. 2A when
the collective voltage is between the supply voltage V.sub.s and a minimum
voltage value 84. Ideally, it has been found that the optimum point to
turn the transistor on is at a phase angle of about 10.degree. past the
midpoint (V.sub.s) while the collector voltage is decreasing. It is
undesirable to turn the transistor on while the collector voltage is
increasing, such as while the voltage is ringing up from the minimum point
84 in FIG. 2A. FIG. 3 illustrates one form of a control and sensing
circuit in accordance with the invention which accomplishes this.
As shown, the control and sensing circuit 80 may comprise a pair of
comparators 90 and 92, which sense the collector voltage V.sub.c of switch
18, or some other voltage which is associated with the transistor on their
inverting (-) inputs, and compare the collector voltage to two different
reference voltages, V.sub.L and V.sub.H. Reference voltage V.sub.H is
higher than reference voltage V.sub.L, and the voltages determine a
voltage range during which the switch can turn on. In a preferred
embodiment, the upper and lower reference voltages may be above and below,
respectively, the supply voltage, V.sub.s, which establishes a control
range about the supply voltage for the switch. For example, for military
applications, a supply voltage of 28 volts is common. In this case,
V.sub.L may be set at 27 volts, and V.sub.H may be set at 29 volts, for
example, thereby establishing a 2 volt control range about the supply
voltage.
As shown in the figure, V.sub.H may be supplied to the non-inverting (+)
terminal of comparator 92, and the output of the comparator may be
connected to the base of switching transistor 18, as shown. The lower
referenced voltage V.sub.L may be connected to the non-inverting input of
comparator 90, and, the output of the comparator may be connected through
a resistor, R, to the inverting input-of comparator 70. The inverting
input of comparator 70 is also connected through a capacitor, C, to
ground, and to another resistor 94 connected to the supply voltage, as
shown. As will be described shortly, the input voltage to the inverting
input of comparator 70 is a ramp voltage having a time constant determined
by the value of resistor R and capacitor C. The ramp voltage is used to
control the on-time of transistor switch 18, as will be described. FIGS. 4
A-D illustrate voltage waveforms at various points in the converter
circuit 60 of FIG. 3, and are useful for explaining the operation of the
invention. FIG. 4A shows the collector voltage V.sub.c of the transistor
switch. FIG. 4B illustrates the ramp voltage V.sub.ramp on the inverting
input of comparator 70; and FIGS. 4C and 4D illustrate, respectively, the
output voltages V.sub.OL and V.sub.OH of the under and over voltage
comparators 90 and 92, respectively. When the collector voltage is above
the higher voltage reference level V.sub.H, the output voltage V.sub.OH of
the overvoltage comparator 92 is low, thereby keeping the base voltage
V.sub.b on transistor 18 low and maintaining the transistor turned off.
Thus, as shown in FIG. 4, between times t.sub.o and t.sub.1, when the
collector voltage V.sub.c is above, V.sub.H, the output voltage V.sub.OH
of comparator 92 is low and, consequently, transistor 18 is turned off.
During this period of time, the output voltage V.sub.OL of comparator 90
is also low, as shown in FIG. 4C, since the collector voltage is higher
than the lower reference voltage .sub.VL. At time t.sub.1, the collector
voltage V.sub.c goes below V.sub.H as shown in FIG. 4A, thereby causing
the output voltage V.sub.OH of comparator 92 to go high, as shown in FIG.
4D. However, the output voltage V.sub.OL is also low at this time and
remains low until time t.sub.2 when the collector voltage drops below the
lower referenced voltage V.sub.L. As long as the output voltage of
comparator 90 is low, this prevents the capacitor C from charging and
maintains the ramp voltage V.sub.ramp low on the inverting input of
comparator 70, which prevents transistor 18 from turning on. At time
t.sub.2, when the collector voltage drops below the lower reference
V.sub.L, the output voltage V.sub.OL of comparator 90 goes high and
capacitor C begins to charge. This produces a ramp voltage V.sub.ramp
(shown in FIG. 4B) to comparator 70. This causes the output of comparator
70 to go high, and, accordingly, the base voltage V.sub.b to go high,
turning on switching transistor 18. When the transistor turns on, the
collector voltage goes low, allowing current to flow through the primary
of transformer 12. Transistor 18 conducts until time t.sub.3, at which
point the ramp voltage becomes equal to the output control signal,
V.sub.reg, from voltage regulator 64. At this time, the output of the
comparator goes low, turning transistor 18 off. This allows the collector
voltage to rise quickly to a level substantially higher than the supply
voltage, typically about twice the supply voltage, as shown in FIG. 4A.
The collector voltage then begins to decrease, as shown. At time t.sub.3,
as the collector voltage rings up above both V.sub.L and V.sub.H , the
outputs of comparators 90 and 92 both go low, as shown in FIGS. 4C-D. When
V.sub.OL goes low, this discharges capacitor C.
Between times, t.sub.3 and t.sub.4, the collector voltage decreases, as
shown. At t.sub.4, when the collector voltage goes below the higher
reference voltage V.sub.H, the output from the over voltage comparator,
92, goes high, as shown in FIG. 4D. However, since the output of
comparator 70 is still low, this holds the base voltage V.sub.b low,
preventing transistor 18 from turning on. At time t.sub.5, when the
collector voltage decays to the lower reference voltage V.sub.L, the
output of the undervoltage comparator 90 goes high, allowing capacitor C
to begin to charge and turning on transistor 18. As the capacitor charges,
the voltage ramp at the input of comparator 70, maintains transistor 18
conductive, as previously described, until the ramp reaches the output of
the voltage regulator, at which time the transistor is turned off and the
cycle repeats.
As previously explained the resonant frequency of the switching power
convertor determines the rate of decay and the ringing characteristics of
the time-varying periodic voltage at the collector of the transistor. The
resonant frequency, as previously described, is determined in part by the
equivalent inductances and capacitances in the primary and secondary
circuits of the power supply, and is a function of a number of different
variables, including the load, temperature and the supply voltage.
However, as can be seen from the foregoing, the invention, by sensing the
time-varying current through the transformer primary winding, as reflected
by the collector voltage, and by controlling the switching transistor as a
function of the collector voltage, the invention enables power to be added
to the primary at the optimum point in the decay cycle and at the optimum
time to ensure high efficiency. Moreover, the invention automatically
compensates and adjusts the switching of the transistor to track changes
in resonant frequency in real time, since any changes in resonant
frequency are reflected as a change in the decay rates and ringing
characteristics of the transistor collector voltage. Thus, the invention
automatically tracks the resonant frequency and corrects its frequency of
operation in accordance with changes in resonant frequency. This is
illustrated, for example, in FIG. 4A, where, at time t.sub.6 it is assumed
that the resonant frequency increases, thereby allowing faster ringdown
and decay between t.sub.6 and t.sub.7 of the collector voltage. This
allows the transistor to turn on in a shorter period of time, at t.sub.7,
than it did in the previous cycle between t.sub.3 and t.sub.5. Thus, the
switching frequency of operation increases as the resonant frequency
increases. Moreover, since the actual collector voltage is monitored and
the transistor switching is controlled at predetermined voltage levels,
the invention can insure that the transistor is always turned off at the
optimum time to avoid damage or destruction of the transistor and to
insure that the appropriate amount of power is supplied to the transformer
primary with the optimum efficiency. Therefore, the invention can operate
at a substantially higher frequency than conventional fixed frequency
switching power convertors which must set their operating frequencies low
enough to insure that any ringing in the collector circuit has died out
before the transistor is turned on. Thus, the invention enables
substantially smaller inductors and capacitors than conventional circuits,
and this permits a reduction in size of weight by a factor of five or
more. As can also be seen from the foregoing, the invention by using feed
forward techniques, insures a faster response than conventional circuits,
and the circuit will always start whenever the control point is higher
than the ramp voltage at comparator 70, since transistor 18 will always
turn on.
FIGS. 5, 5a and 5b illustrates another embodiment of a self-compensating
resonant frequency switching power convertor in accordance with the
invention. The power convertor of FIG. 5 is substantially similar to the
embodiment of FIG. 3, but advantageously provides for self-compensation of
the converter with varying supply voltages. This can be a substantial
advantage in, for example, mobile operations as encountered typically in
military applications. In such cases, a nominal 28 volt supply may vary
between 22 to 38 volts. The power convertor of FIGS. 5a and 5b
automatically compensates for changes in supply voltage by automatically
adjusting its reference voltages and operating point.
Referring to FIG. 5a where like reference numerals are used to designate
similar components to those in FIG. 3, the lower and higher reference
voltages V.sub.L and V.sub.H applied to the under and over comparators 90
and 92, respectively, may be derived from a voltage divider network
comprising resistors 100, 102, and 104, connected in series between the
supply voltage and ground. A second voltage divider comprising resistors
106 and 108 connected in series between the collector voltage of
transistor 18 and ground are used to derive a sample of the collector
voltage for comparators 90 and 92. By selecting the values of the
resistors 100-108 in the two voltage divider networks, the relative
operating voltages of the two comparators can be established and
referenced to the supply voltage V.sub.s. If, for example, the supply
voltage is 28 V, nominally, the collector voltage V.sub.c in a quiescent
state with the transistor off would be equal to the supply voltage of 28
V. By selecting resistors 106 and 108 to be of equal value, the reference
level for the collector voltage to the comparators 90 and 92 can be set,
for example, at 14 V, nominally. The values of resistors 100-104 are
similarly selected to provide the desired high and low reference voltages
to the over and under comparators. If, for example, resistors 100 and 104
are made equal in value, and resistor 102 is selected to have a value
which is relatively small compared to that of resistors 100 and 104,
resistor 102 establishes a small dead zone which determines the
differential between the upper and lower referenced voltages V.sub.L and
V.sub.H. For example, assuming that supply voltage is 28V, by selecting
resistors 100 and 104 to be 1 Kohm, and resistor 102 to be 100 ohms, the
higher voltage reference V.sub.H to comparator 92 will be 14.6 V, and the
lower voltage reference V.sub.L to comparator 90 will be 13.3V,
establishing approximately a 1.3V range between the upper and lower
reference voltages. Moreover, since the reference voltages to comparators
90 and 92 are derived by the two voltage divider networks as a portion of
the supply voltage, any variations in the supply voltage would be
reflected as variations in the absolute values of the reference voltages.
However, their relative values would remain constant, thereby enabling the
operating point of the circuit to automatically compensate for changes in
supply voltage. This stabilizes the circuit and provides faster correction
in operating point. Moreover, since the ramp voltage to comparator 70 is
also referenced to the supply voltage, if the supply voltage increases,
this will change the slope of the ramp voltage, thereby varying the
charging rate of capacitor C to charge it faster. This decreases the
transistor on-time, which is appropriate, since with a higher supply
voltage, the transistor does not need to be on as long to transfer a given
amount of power to the transformer.
The connecter circuit of FIGS. 5a and 5b may also include a variable
resistor 110 on the input to voltage regulator 64, which conveniently
allows the output voltage from the power supply to be adjusted by
adjusting the control voltage to the ramp comparator 70. A Schott key
diode 112 connected between the output of the voltage regulator 64 and the
comparator 70 insures fast pull-down of the ramp voltage upon switching of
the transistor. In addition, The circuit of FIGS. 5a and 5b may include an
overcurrent limiter comprising a comparator 114 which senses the current
flow in a sense resistor 116 connected between the emitter of switching
transistor 18 and ground. When the current flow through the sense resistor
116 exceeds a predetermined value, determined by the setting of a
reference voltage to comparator 114 by a variable resistor 118, the output
of the comparator goes low, which turns off the transistor. This is
advantageous for preventing destruction of the transistor in the event of
a short circuit or other malfunction.
Other differences in the circuit of FIG. 5 include a pair of transistors
connected as a differential driver 124 between comparator 70 and the base
input of switching transistor 18, and a voltage multiplier comprising a
plurality of capacitors 130 and diodes 132 to step up the output voltage
at the secondary of the transformer. High voltage transformer 12 may
comprise, for example, a 100:1 step-up ratio. Other additions may include
filter capacitors 140 and an inductor 142 on the supply voltage line, and
bypass capacitors 144 around various ones of the resistors in the divider
networks which improve noise performance.
In other embodiments of the invention, the high voltage transformer may be
replaced, for example, by a piezoelectric ceramic transformer, for
example, of PZT-8 ceramic or lithium niobate. Piezoelectric transformers
can carry very high voltages, and have the advantage of being small and
lightweight. By varying the length to width ratio of a bar of a ceramic
piezoelectric material, a voltage ratio of 1000 times can be easily
achieved.
Also, a different form of sensing and control network 80 illustrated in
FIGS. 3, 5a and 5b may be employed. As indicated earlier, the optimum time
to apply power to the primary of the convertor transformer is when the
voltage transient is decreasing below the supply voltage during its
ringdown. Thus, the sensing and control circuit 80 could be replaced with
a slope detector, e.g., a differentiator, which detects a decreasing
voltage in combination with a voltage level comparator so that the
switching transistor is turned off at a point in its operating cycle where
its collector voltage is within a predetermined voltage range, such as
region 83 shown in FIG. 2.
From the foregoing, it can be appreciated that the invention affords
switching power convertors having significant advantages over known
fixed-frequency switching convertors. The invention can operate at
substantially higher frequencies than conventional convertors, for
example, 300 kHz or higher, as compared with an operating frequency of the
order of 30 Khz for conventional switching regulators. This results in
significantly smaller size and lighter weight, since large inductors and
capacitors are not required to insure a low operating frequency as in
conventional switching regulators. This makes the invention ideally suited
for portable devices, such as for powering field emission displays or the
like in laptop computers. Moreover, the invention can operate at low
battery voltages of the order of 3.3 V as is typical of some of the newer
integrated circuit technologies. The invention also is applicable for
other high voltage applications such as powering traveling wave tubes,
cathode ray wave tube displays, or x-ray tubes, as well as in applications
requiring small size and lightweight as on military aircraft.
While the foregoing has been with reference to particular embodiments of
the invention, it will be appreciated that changes in these embodiments
may be made without departing from the principles and the spirit of the
invention, the scope of which is defined in the appended claims.
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